The long-term objective of our research continues to be a better understanding of the molecular mechanism of action of hormones on hepatic gluconeogenesis. We have put forth the general hypothesis that glucagon, insulin, and catecholamines act in the pathway by affecting the phosphorylation state of several enzymes involved in the substrate cycles between pyruvate and phosphoenolpyruvate and fructose diphosphatase and fructose-6-phosphate. The regulation of pyruvate kinase, fructose diphosphatase and phosphofructokinase is emphasized since all of these enzyme have shown to be phosphorylated by protein kinases in vitro with associated alteration in enzyme activity. The regulation of pyruvate kinase and fructose diphosphatase activity by phosphorylation-dephosphorylation will be related to hormonal control of gluconeogenic flux. The phosphorylating and dephosphorylating enzyme involved in such control will be identified and characterized and their hormonal control studied.